1. Field of the Invention
The present invention relates to an electrode cable of the type having a plurality of wires arranged helically along the length of the cable, said cable, which is encased in an outer, tubular sheath made of an electrically insulating material, suitable for use as an electrical connection between an electrical stimulation device, such as a heart stimulator, defibrillator etc. connectable to the proximal end of the cable, and an electrode connected to the distal end of the cable.
2. Description of the Prior Art
A lead device having one or more helices, each formed by at least one conductor, is known from European Application 0 162 178. Each conductor is formed by a number of wires arranged in a bundle by intertwining. All the wires in each conductor can be made of the same or of different materials. When different materials are used, the wires can differ in strength and electrical conductivity. This known lead device can be used as an electrical connection between e.g. a heart stimulator and contact electrodes for implantation in the human body. The objective of this known lead device is to achieve a device which, with the smallest possible external diameter for the helix, displays great fatigue resistance, relatively great electrical conductivity and small electrical resistance.
Each helical conductor in this known lead device has a core made of a wire and a plurality of wires coiled or wound around that core""s longitudinal axis. The wires helically coiled around the core are intertwined into a bundle, making the lead device rather stiff and, therefore, scarcely suitable for advancement through the vascular system together with a stylet unit, inserted into a central channel in the lead device, to a desired location in the heart daring an implantation.
Another lead device having one or more helices that is similar to the above device is disclosed in U.S. Pat. No. 5,483,022. The difference is that each wire in the plurality of bundled wires is a wire having a conductive metal core jacketed in a sleeve of a less conductive metal having a higher strength and a better biocompatibility. One of the wires serves as a core similarly to the core in the above device.
An objective of the present invention is to provide a low-resistivity electrode cable which can be used as an efficient electrical connection between a device for electrical stimulation in the human body and an electrode connected to the device. The device can be e.g. a heart stimulator or defibrillator, and the electrode can be e g. an implantable heart electrode. The electrode cable can also be used for nerve stimulation. The electrode cable must be designed so it has very good electrical conductivity while simultaneously displaying optimal strength, tensile strength, flexural strength and fatigue resistance in particular.
In other words, the problem addressed by the invention is to provide an electrode cable with optimal properties in respect to its electrical conductivity, mechanical strength and ability to withstand dynamic stress capable of causing fatigue breakage of the cable, all of which are events which could have fatal consequences for a patient with a heart stimulator.
Another objective of the invention is to provide an electrode cable design making it possible to minimize the use of expensive materials in manufacturing the wires or conductors in the cable.
The problems cited in conjunction with the above objectives have conventionally been addressed to date by the use of either xe2x80x9chigh-resistivityxe2x80x9d electrical conductors ( greater than 60xcexa9 for a given reference length) or xe2x80x9clow-resistivityxe2x80x9d conductors ( less than 15xcexa9 for the same reference length) with a cylindrical spiral (helical) shape, a number of conductors wires (also referred to as wires) being helically wound or intertwined with each other.
In the use of such high-resistivity conductors, total resistance can obviously be reduced by e.g. increasing the total cross-sectional area of the conductive material in the lead, i.e. by increasing the near of wires in the lead and/or increasing the cross-section of each wire in the lead. However, an increase in the number of wires in the lead increases the lead""s external diameter which is a disadvantage for a patient in whom such an electrode is to be implanted. The disadvantage is even greater if a plurality of leads must be implanted in the patient. However, an increase in the cross-sectional area of a conductor has an adverse impact on e.g. fatigue resistance. When the number of wires in a helical spiral increases, the pitch of each individual conductor wire also increases, thereby impairing the mechanical properties of the lead.
When the aforementioned type of low-resistivity conductors are utilized, the conductors employ wires made of either a low-resistivity material or a material which is a combination of a low-resistivity material and a high-resistivity material, the latter devised to carry the low-resistivity material. However, such low-resistivity wires have limiting mechanical properties.
The above objects are achieved in an electrode cable according to the invention containing at least a first set of electrically concussive wires, this first set of wires, (and each additional set of wires, if present) including at least one wire devised as a low-resistivity electrical conductor and at least two or three, wire(s) devised as (a) high-resistivity electrical conductor(s), and wherein all the low-resistivity and high-resistivity wires in each set of wires are conductors with the same diameter and are arranged side-by-side in a strip running parallel to the exterior of the cable sheath and extending helically along the cable""s length. In a preferred embodiment all wires are conductors with the same diameter in the 0.05 to 0.20 mm range.
The following can be noted to clarify the concepts xe2x80x9clow-resistivityxe2x80x9d and xe2x80x9chigh-resistivityxe2x80x9d wires (conductors) in the present application. These two concepts are designations whose primary purpose is to indicate that resistivities with clearly differing magnitudes are involved. The ratio between them is particularly important in this context. As is well-known, resistivity is an electrical ,unit measured in xcexa9m. The following are examples of magnitudes and ratios applicable to low-resistivity and high-resistivity wires respectively.
A example of a low- resistivity wire is a 5.8 m long wire with a cross-sectional area of 0.00785 mm2 in which a low-resistivity material (silver) constitutes 28% of the conductive material and a high-resistivity material (MP35N) constitutes 72%. This results in a wire (conductor wire) with resistivity on the order of 5.4xc3x9710xe2x88x928 xcexa9m. A high-resistivity wire with the exact same geometry as the low-resistivity wire but made only of the high-resistivity material (MP35N) will have a resistivity on the order of about 2.5xc3x9710xe2x88x927 xcexa9m. The ratio between the resistivity of the high-resistivity wire and the low-resistivity wire according to this example will amount to about 4.6.
With this kind of hybrid helical wire spiral with one or two low-resistivity wires as the primary electrical conductor means, high-resistivity wires serving as secondary electrical conductor means, providing support for the low-resistivity wires and possessing good mechanical properties, a low-resistivity helical wire can be achieved, which also displays good mechanical properties.
In one preferred embodiment of the electrode cable according to the invention, wire connecting means are arranged on each cable end and accomplish electrical interconnection of all the wires in the respective wire set, thereby achieving their parallel connection in the cable. In this manner, a low-resistivity helical wire is obtained with good mechanical properties.
The wire connecting means on each cable end can e.g. be some appropriate type of metallic clamping means which achieve surface contact with each low-resistivity wire and one or more of the high-resistance wires. The clamping means can also be made of a pair of interacting parts which grip in/on the wire spiral. Alternately, the wire connecting means can consist of wire-connecting welded or soldered joints on the ends of the cable connecting the wires.
The high-resistivity wires in each wire set are conductors made of the same material, and the low-resistivity wire in each wire set is a wire conductor with a core of low-resistivity material (e.g. silver, gold, copper, platinum etc.), the core being encased in an external jacket made of the same kind of material as the material in the high-resistivity wires. The latter material could be e.g. a cobalt alloy such as MP35N or the like. Material in the external sheath of each low-resistivity wire comprises appropriately 65% to 80% of the wire""s total volume. It has been shown that particularly good properties are achieved when the latter percentage amounts to about 72%.
With the invention, it also becomes possible to minimize the use (i.e. the required quantify) of a corrosible material (e.g. silver, copper etc.) in the electrode cable. The low-resistivity material risks becoming corroded or forming a cathode, the surrounding material becoming anodic, which may cause corrosion. In the electrode cable of the invention, the amount of mixed material can be reduced, thereby increasing the percentage of material such as MP35N, thus also improving the conditions for obtaining an effective welded joint.
The electrode cable according to the invention can be a unipolar cable, but the electrode cable can alternately contain additional electrical conductors (poles), the cable thus becoming bipolar or multipolar. An electrode cable devised as a bipolar cable according to the invention can contain a second set of helical wires forming the cable""s second pole. In one embodiment, the inventive bipolar electrode cable is hollow throughout its length, having a longitudinal channel into which a stylet unit can be inserted. The stylet unit is temporarily inserted into the electrode cable during cable implantation to facilitate advancement of the cable through the vascular system to the desired location in the heart.